1. Technical Field of the Invention
This invention relates generally to communication systems and more particularly to devices within such systems.
2. Description of Related Art
A civilian-use global positioning system (GPS) includes a constellation of synchronized satellites in medium earth orbit that transmit precise wireless signals. The wireless signals are transmitted utilizes a frequency of 1.57542 GHz and a code division multiple access (CDMA) modulation scheme. Each wireless signal includes a timestamp of when the signals are sent, orbital information of the satellites, and a unique pseudo-random sequence code.
A GPS receiver needs to receive at least four precise wireless signals from four different GPS satellites to determine its location, which includes an altitude component. If the altitude of the GPS receiver is known, then it can determine its location from three signals from three different GPS satellites. Assuming the GPS receives is receiving a sufficient number of signals, it determines it location by calculating transit times for each of the received signals (e.g., the time difference between the timestamp and when the signal is received). The GPS receiver then determines, based on the transmit times, a series of distances between itself and the corresponding satellites (e.g., the ones from which the receiver received signals). The GPS receive then calculates it location based on the series of distances using a geometric trilateration function.
While the orbit of the GPS satellites are arranged such that approximately six to ten satellites are visible with line of sight from almost anywhere on the Earth's surface, there is little margin for error. For instance, a typical GPS receiver operates with very little link margin (e.g., 5-10 dB) since it has to overcome a path loss of approximately 180 dB (e.g., decrease of signal strength from the transmitting satellite and a GPS receiver). When the GPS receiver is inside a building, its link margin is typically more than consumed by the building's penetration path losses of 5-15 dB. As such, when a GPS receiver is inside the building, it typically does not receive a sufficient number of GPS signals to determine its location.
To address the loss of a GPS receiver's ability to determine its location in a building, industry has developed several non-GPS based in-building location techniques. Such techniques include time difference of arrival (TDOA), cellular site (identification) ID, gyroscope and accelerometer, wireless RF beacons (RF-ID tags), infrared beacons, ultrasonic beacons, wireless signal strength, and angle of arrival (AOA).
The TDOA technique can locate an object in two dimensions (2D) by calculating the time difference between the receiving of three signals transmitted by three synchronized transmitters at fixed locations. The receiver's location can be calculated in three dimensions (3D) if a fourth transmitter or receiver is added. While dedicated TDOA systems address the in-building issues, they require special synchronized equipment that can be expensive. In addition, accuracy of a TDOA system may be an issue when the transmitters are on the outside of a building and the receiver is on the inside, which is amplified if floor-by-floor resolution is desired.
The cellular site ID technique broadcasts a unique cell site ID message from each cell transmitter (e.g., common to all sectors in a cell or unique to each sector in each cell). Cellular receivers receive the unique ID to determine their location. While cellular site ID systems address the in-building issues, their resolution may be an issue since the area within a sector or cell can be quite large (e.g., one mile).
The gyroscope and accelerometer technique utilizes a combination of one or more of gyroscopes and accelerometers to track the motion of a portable device. While gyroscope and accelerometer technique addresses the in-building issues, its accuracy may be an issue as the measurements drift over time, which cause errors to accumulate.
The beacon technique (e.g., wireless RF, infrared, ultrasonic beacons) sprinkles low power transmitters through a desired area where the transmitters substantially do not have overlapping coverage. Each beacon periodically transmits a unique identifier, which is received by a portable device. The device utilizes the unique identifier to determine its relative location. While the beacon technique addresses the in-building issues, it can be costly since it requires many special purpose transmitters be installed in the desired area.
The AOA technique and the wireless signal technique locate an object based on signals that are transmitted by multiple transmitters at known locations. The AOA technique calculates the angles of arrival of the signals and the wireless signal technique calculates the signal strength of the signals. From the angles or signal strengths, the location of the receiver can be determined. While these techniques address the in-building issues, they do so at the cost of extra equipment. Further, if the transmitters are on the outside of a building and the receiver is on the inside, then multipath reflections and path loss adversely affect the receiver's ability to receive the signals.
Therefore, a need exists for a communication device that addresses one of more of the in-building location issues.